What Is Meant By A “Bank” Of Chilled Water Coils

For those that work with HVAC installations on a regular basis, you have run across the problem of needing to install new chilled water coils in very tight, confined areas. The coil is too big to fit in the Chilled Water Coilselevator, and/or the HVAC room is so small that you are likely to damage the coil simply by moving it. As a solution to this challenge, chilled water coils are often installed in “banks” of coils. You are most likely to see this configuration in Air Handler Units, as well as “built-up” systems. Due to face velocity limitations across the coil, you will need larger coils in order to meet your required face area. With this in mind, there are a few specific reasons why you want to avoid having a single, large coil in one of your units.  Starting with the obvious: larger coils are much more difficult to transfer and install. This is especially true for older buildings, where the rooms were essentially built around the HVAC system.

As you’ve probably experienced, some of these areas can barely fit a single person, so installation – if even possible – is a logistical nightmare. Also, the larger the coil, the easier it is to damage during transport to the jobsite. To avoid these issues, simply break down the single, larger coil into smaller coils. When piped together, those smaller coils are stacked into “banks” of coils in the system. If installed correctly, this “bank” should have the same performance as the larger, single coil.

Casing

There are many different casing options available, but “stackable” flanges are required for heavy chilled water coils that are “banked”. The flanges are often inverted inward and down to give added strength to the casing, which is needed due to the fact that another coil of equal weight will be stacked on top of it. When ordering coils in a “bank” configuration, be sure to let the manufacturer know that they will be “stacked”.

Many engineers also use stainless steel casings on chilled water coils. While more expensive than traditional galvanized steel, stainless steel protects against excessively wet coils and/or corrosive elements in the airstream. Keep in mind that the majority of coils fail because of old age and its casing, as opposed to failure with the coil’s core. With that in mind, doesn’t it make sense to select heavy-duty stainless steel casings that are more durable and meant for stackable installations?

Drain Pans & Water Carryover

Water Coils

All chilled water coils must be sized so that the face velocity across the coil does not exceed 550 ft/minute. Water on the outside of the coil is carried away from the coil’s leaving air side in an arc, while water in the highest point of the coil is carried further down the unit or ductwork. “Stackable” coils often require intermediate drain pans under each coil to catch the excess water carryover. Each coil in a bank requires its own drain pan, as a single, large pan under the bottom coil is not enough.

Circuiting/GPM

If all of the coils in a “bank” are of equal size and handling the same CFM, then the GPM of each coil will also be the same.

Always feed the bottom connection on the supply header on the leaving air side of the coil. This ensures counter air and water flow. This also prevents the coil from short circuiting because the header fills first and circuits all of the tubes equally.

Designing Banks Of Coils

Almost all coil “banks” perform more efficiently if you design something more square in shape, as opposed to long and/or high. In a “bank” of coils, you may find that one coil has points of 300 ft/minute, with other points at 800 ft/minute. Scenarios such as this will cause water-carryover! You generally want to be as close to 550 ft/minute as possible in order to allow equal airflow distribution across the face area of the coil.

Anytime you are designing and/or building coils, work closely with the manufacturer as an added resource to ensure that you are getting the ideal solution for your HVAC system. Capital Coil & Air works on similar jobs such as these daily, and we welcome the opportunity to work with you in whatever capacity is needed.

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Tips on Hand Designation & “Counter-flow”

Are your chilled water coils right hand or left hand?  Are you looking into the face of the coil with the air hitting you in the back of the head?  What exactly is counter-flow and why is it important?  Are you completely confused by why right hand vs. left hand even exists?  Most manufacturers probably do not know or understand the technical reasons themselves.

First, let’s figure out what coils even need a hand determination.  Chilled Water Coils, Direct Expansion (Evaporator) Coils, and Condenser Coils are the only coils that need this figured on almost every job.  Hot Water Coils, Booster Coils, and Steam Coils rarely need this determination!  The reason for this is when the coils are only 1 or 2 rows deep, they can be flipped over.  When a chilled water coil is 3+ rows deep, hand determination is much more important because it needs to be counter-flow.  With most suppliers determining hand designation with the air hitting you in the back of the head….do you want the connections on the right or left?

Chilled Water CoilsYou’ve probably heard the term “counter-flow” countless times, but here’s the simplest explanation.  For peak performance, you want the air and the fluid traveling in opposite directions through the coil.  Is it the end of the world if your coils are not counter-flow?  The short answer is no, but you will lose anywhere from 12-15% of the output.  So if your coils are piped incorrectly, don’t expect to get the full performance.  Steam and hot water coils are 1 or 2 rows deep, so again, counter-flow is pretty much irrelevant.  However, it can make a BIG difference with any chilled water or direct expansion coils (3-12) rows deep.

We also get asked many times “what is the proper way to pipe coils?”  Put simply, steam coils should always be fed on the highest connection and the return on the lowest connection.  Water coils should always be fed on the lowest connection and returned on the top connection to ensure that all of the tubes are are fed the same volume of fluid. 

Hand designation and counter-flow are two pretty simple concepts when they are properly explained.  When dealing with a HVAC coil manufacturer, partner up with one who will walk you through the engineering and explain it along the way.  Capital Coil & Air has well over a decade of experience in handling pretty much any scenario that you may come across, so we want to be your coil resource for any and all projects. Please give us a try on your next job!

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What Does “Splitting” A DX (Evaporator) Coil Mean?

“Splitting” a DX (Evaporator) coil is one of the toughest concepts to understand in the coil business. “Splitting” the coil simply means that (2) compressors can operate off of the same coil. One obvious advantage, or reason that you might “split” a DX (Evaporator) coil is that you can shut down (1) of the compressors when the cooling load does not require it. This in turn saves energy, which saves $ when the cooling load is not operating at maximum design conditions. For example, let’s use a coil that is designed to give you (40) tons, but the coil is split so that (2) 20-ton compressors are feeding the same coil. If you only require ½ of the maximum load on any given day, you can shut down (1) compressor completely and operate the other one at 100%. This is a money-saving feature that you need to be aware of if you deal with DX coils on a regular basis. This requires special circuiting arrangements, and this is where the confusion starts with most folks. There are three primary ways to deal with this:

FACE SPLIT

Splitting the coil is nothing more than putting (2) completely separate fin/tube packs (coils) into one common casing. When you hear the term “face-splitting” a coil, you are drawing a horizontal line from left to right across the face of the coil and dividing the coil into a top and bottom half. It is like having two separate coils in one casing in that each half is circuited by itself. You hook up (1) compressor for the top half, and (1) compressor for the bottom.

In practice, this configuration is no longer used with much frequency because this arrangement leads to air being directed across the entire face of the coil. This disadvantage is especially apparent when only one half of the coil is in use because you’ll need a complicated damper/duct system to ensure that air is only directed to that portion of the coil in operation.

Row Split

“Row splitting” a coil is dividing the coil by drawing a line vertically and putting some portion of the total rows in (1) circuit, while putting the remaining rows in the other circuit. With this configuration, the air passes across the entire face of the coil, and will always pass across the rows that are in operation.

Please be aware that this configuration also comes with certain issues in that this kind of split makes it very hard to achieve a true 50/50 split. Let’s use an (8) row coil as an example. You would like to “row split” this coil with (4) rows/circuit, which would appear to be a perfect 50/50 split. The problem here is that the first (4) rows, located closest to the entering air, pick up a much higher portion of the load than the last (4) rows. In actuality, this coil’s split is closer to 66% / 34%, which will not match the 50/50 compressors. Another option is try to split the coil between (3) & (5) rows. While not 50/50 either, this configuration is closer. However, a new challenge arises because you have now created a coil that is very difficult to build and correctly circuit. In short, you need almost perfect conditions along with a degree of luck to achieve a true 50/50 split using this method.

Intertwined Circuiting

The most common to split coils today is to “intertwine” the circuiting. This means that every alternate tube in the coil is included in (1) circuit, while the other tubes are included in the (2nd) circuit. For example, tubes 1, 3,5,7,9, etc. in the first row are combined with tubes 2, 4, 6,8,10, etc. in the second row. The same tubes in succeeding rows form (1) circuit. You are essentially including every alternate tube in the entire coil into (1) circuit, which (1) compressoDX (Evaporator) Coilsr will operate. All of the remaining tubes not included in the first circuit will now encompass the second circuit.

The advantage of this configuration is that the air passes across the entire face of the coil, and, if one of the compressors is on, there are always tubes in operation. Every split is now exactly 50/50 because it cannot be any other way. Most DX coils are now configured in this manner due to these advantages.

Capital Coil & Air has years of experience measuring, designing and building almost every OEM DX coil that you’ll come across, so please let us help you on your next project. We want to be your replacement coil experts and look forward to the opportunity.

 

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Condenser Coils Failing? Here’s probably why….

Did you recently turn on your DX systems only to find your Condenser Coils are not working?  Simple fix right?  Unfortunately, no.  If you get lucky, you can send us the model number of the unit, and there’s a great chance we’ve already built it.  In the case that we do not have that model number on file, you have two options.  You can go back to the OEM, wait (5) months for a part and pay through the roof.  Or you call Capital Coil, and we’ll walk you through the engineering it takes to replace a condenser coil.                                                                      Condenser Coils

Very rarely do condenser coils ever freeze so the first thing you’re going to want to know is if your coil died of corrosion, old age, or possibly vibration.  Old age is obviously preferable because with a few easy dimensions, we’ll have enough to price up your duplicate coil.  Condenser coils are usually outside and are easily accessible for measurements and digital pictures.  With just the size, the rows, and fins/inch, you can get a price.  And digital pictures of the headers and return bends will give us a good idea of the circuiting and sub-cooler circuits. 

If the coil has been eaten away by corrosion, it was an improper design to begin with.  Most people don’t know that salt in the air will ruin aluminum fins within a year or two.  There are two ways to combat this.  The first option is to make the switch to copper fins and stainless steel casings.  While this will extend the life of your coil considerably, most people are not too happy about the additional cost over aluminum fins.  The second option is to use a coating.  Coatings are the much more popular choice.  They are a fraction of the cost as copper fins and only add (1 – 2) weeks to your lead time. 

When your HVAC coils are installed near a moving piece of equipment, vibration can occur and cause leaks.  The area where these leaks occur is very important and will clue you in to if the problem is vibration.  If they are near the tube sheet and look like they are slicing through the tube, the coils should be isolated from the rest of the system to prevent vibration from causing damage.  One way to combat this is by oversizing the tubesheet holes, but many manufacturers will not do this.  Condenser coils are usually the most common victims of vibration.

The last concern is with cleaning condenser coils.  Since condenser coils see outside air almost exclusively, they need to be cleaned more than other coils.  The reason for this is most condenser coils have fin spacing of 12-20 fins/inch.  With fins that tight together, the coil can and will act like a filter.  And when the coil is clogged up, the performance suffers greatly.  Recently, we’ve been getting more and more calls about using a heavier fin thickness.  This is to help with high pressure cleaning and corrosive cleaning agents. 

When dealing with an HVAC coil manufacturer, partner up with one who will walk you through the engineering and explain it along the way. Capital Coil & Air has well over a decade of experience and has seen every issue to make sure your everything from the quote to the installation go smoothly! Give us a try on your next project!

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Coils and Counter-flow: 5 Common Questions

1)  Coils and counter-flow?

The first thing to remember about coils and counter-flow is that chilled water coils are always built to be piped in counter-flow. This means that the air flows in the opposite direction as the water. For example, with counter-flow, the air flows through rows 1-8, while the water runs through rows 8-1. Water always travels through the coil in the opposite direction of the air; hence the term “counter-flow.”  Direct Expansion Coils (Evaporator Coils) are also piped in the same manner.

With that said, what happens when you do not pipe cooling coils counter-flow? Almost all coil selection programs you will see or use will be based on counter-flow conditions. If you opt to not counter-flow a chilled water coil, you’ll have to reduce the coil’s overall performance by a certain percentage. That percentage reduction varies based on each coil’s unique dimensions, but a reliable estimate is a loss of 8-12%. Simply piping the coils in the correct manner from the beginning would seem to be the easiest and most cost-effective solution.

2)  Why do you feed from the bottom of the coil?

DX Coils

You always want to feed a water coil from the bottom connection so that the header fills from the bottom on up and feeds every tube connection evenly. All tubes must be fed evenly with the same amount of water. If you try to feed the header from the top, you greatly increase the risk of “short circuiting” the coil and having a higher water flow through the top tubes in the coil.

3)  What is a Water Hammer in a Steam Coil?

On a long Steam Coil, you will be hard pressed to get the steam through the length of the coil. Slowly but surely, that steam converts into condensate, which is pretty much the worst thing that can happen to any system. If not evacuated, the condensate just lays in the coil when the system is shut off. This problem comes into play when the steam is turned back on and meets the condensate laying inside the coil. In addition to the noise, the steam and condensate cause huge amounts of additional stress on the coil’s joints. As a result, over time, your coil will inevitably fail.

4)  What else happens if you do not evacuate condensate?

When you cannot or do not evacuate the condensate on long steam coils, the condensate ends up blocking the steam. A steam coil should never feel cool to the touch, but when condensate blocks steam, one part of the coil will be warm while the other will be cool. Again, that should not happen. Steam coils are interesting in that they are more dependent upon the system and installation than any other type of coil. A steam coil must be pitched to the return end of the coil. Obviously, steam is not water. Traps, vacuum breakers and other steam accessories must be installed and located properly for the system to function.

5)  Is it necessary to pipe steam and/or hot water coils in counter-flow?

Simply put – no! Circuiting a coil is only necessary to ensure the connections are on the side of the coil that you want. The rows and tubes in the coil dictate how and where you feed, but the steam supply always needs to be the high connection. This method ensures that the leaving condensate is on the bottom of the coil and below the lowest tube within the coil. Whatever else you do, know that the condensate must leave the coil!

If you have any questions or need assistance with ordering and/or installation, please contact a sales engineer at Capital Coil & Air. We will walk with you step-by-step through your entire project should you require any assistance. CALL OR E-MAIL US!  We look forward to the opportunity to work with you on your future projects.

 

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10 Things To Know About Chilled Water Coils

Chilled Water Coil

1. Hot or chilled water coils are still water coils. There is really no difference between hot water coils and a chilled water coils in construction. Hot water coils are usually 1 or 2 rows and chilled water coils are usually 3 to 12 rows deep.

2. The vast majority of chilled water coils are constructed from either 1/2″ OD tubes or 5/8″ OD tubes. A lot of that depends on the tooling of the original equipment manufacturer and what is more economical. Either size can be used and substituted for each other, which makes replacing your coil that much easier.

3. 1/2″ Tubes are on 1.25″ center to center distance. 5/8″ tubes are on 1.5″ center to center distance. For example, if a chilled water coil has a 30″ fin height, there will be (24) 1/2″ tubes per row or (20) 5/8″ tubes per row. The tube area of the coil is remarkably the same. They are almost interchangeable.

4. The quality of the coil often times is directly tied to the tube thickness. Many installations have water not treated properly or tube velocities that are too high. There are few perfect installations in real life. Increasing the tube wall thickness on a chilled water coil is a great way to ensure longer life.

5. Fins make great filters! Of course, they are not designed to be filters, but it happens. You can make any coil cheaper by making them 14 fins/inch with less rows rather than 8 or 10 fins/inch. Just remember that deep coils are very difficult to clean. Cheap is not the way to go most of the time!

6. Fins are designed for maximum heat transfer. They are much more complicated in design than they appear to be when looking at the chilled water coil. They are rippled on the edge to break up the air. They are corrugated throughout the depth of the fin. The tubes are staggered from row to row and the fin collars are extended. All of this to maximize heat transfer. Unfortunately, the byproduct of this is the fins can end up being great filters. Be careful in the design of any chilled water coil.

7. Fins are aluminum for a reason! They give you great heat transfer at an economical cost. You need a compelling reason to switch to copper fins as copper is very expensive, and you’re likely to double (or maybe triple) the cost of the coil. Coatings are popular for this very reason.

8. Many chilled water coils are built with 304 stainless steel casings. The casings are stronger, they last longer, they are stackable, and it’s fairly inexpensive. After all, what is the point of building the best coil possible and have the casing disintegrate over time around the coil? Sometimes, it’s money well spent!

9. Circuiting the coil is the tricky part of any coil. Circuiting is nothing more than the number of tubes that you want to feed from a header. There are two rules. You must keep the water velocity over 1 foot/second and below 6 feet/second. 3-4 feet/second is optimum. The second is the number of tubes that you feed must divide evenly into the number of tubes in the coil.

10. Replacing  your chilled water coil is easy. Rarely do you have to worry about the performance. When you replace a 20 year old coil, it is dirty and the fin/tube bond is not good. The coil is probably operating at 1/2 of its capacity at best. When you put a new coil on the job, your performance will automatically be terrific. Your main concern is now making the sure the coil physically fits in the space allowed. And always have this in the back of your mind: Smaller is always better than too large. Smaller you can always work with, whereas too large makes for a very ugly and expensive coffee table.

There you have it – everything you need to know about chilled water coils. Interested in learning more, please reach out to Capital Coil & Air! We look forward to the opportunity to be your coil replacement specialists!

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OEM Replacement Coils: Repair or Replace

When considering OEM replacement coils, there are multiple reasons why coils can fail prematurely. Sometimes, OEM Coils simply freeze and can never be repaired. Other times, the coil was selected incorrectly, which in turn, made the coil significantly underperform. Many times, there is substantial corrosion or something else in the system that causes the coil to fail. However, most coils, when selected correctly, and in systems that are properly maintained, can last anywhere from 10-30 years!  10-30 years is also a pretty wide range, and there are many variables in how long you can expect a coil to perform. Factors, such as ongoing maintenance, air quality, and water/steam quality all have an effect on a coil’s lifespan.

OEM Replacement Coils

Reasons Why Coils Fail Of Old Age

  • While the coil’s tubes are considered the primary surface, 70% of all coil performance is performed by the finned area on a coil, which is known as the secondary surface. The fin/tube bond is easily the most important manufacturing feature in any coil. Without the bond between the tubes and fins, the coil could never properly function. Like all things however, over time the fin/tube bond becomes less efficient with constant expansion and contraction. While the construction of the coil, as well as the fin collars, does not allow the fins on the coil to move, that fin/tube bond naturally weakens a coil’s life over time after installation. Because of this, it is not a stretch to say that a coil is easily 30% less efficient after (20) years.
  • Cleaning coils often pushes dirt to the center of the coil, and this occurs even more so on wet cooling coils. Just remember that coils can become great air filters if not properly maintained. The BTU output of any coil is in direct proportion to the amount of air going through the coil. If you decrease the CFM by 20%, you are also decrease the BTU’s by 20%!
  • Cleaning agents often corrode aluminum fins. Since every square inch of fin surface matters in performance, corrosion of the fin surface is always detrimental to the coil’s performance.
  • Many times, there are coil leaks simply because of old age. No coils are immune to erosion. You might find the brazing in the tubes, as well as the brazing in the header/tube connections failing over time. Steam can be both erosive and corrosive under higher pressures. Water travels through the coil at 2 – 5 ft/second, so erosion is an enormous part of coil failure, regardless of how well-maintained. Erosion is always there, whether you realize it or not.
  • Water/steam treatment and the corrosive effects of bad steam/water can all be causes of coil failure…which then necessitates the need for a reliable manufacturer for OEM replacement coils.

So What Is The Solution?

Some coils can last 5 years, and some coils can last 30 years. As you have read, there are numerous factors that contribute to a coil’s life. In the end, there will most likely have been multiple attempts to repair that coil to make it last as long as possible. The depressing news is that most of these “Band-Aid” attempts do not work well. The most likely outcome is that you are buying a new coil anyway, so why waste the time and money on a temporary solution?

Coil failure is a “pressure event”, which is a fancy way of saying that a coil is leaking. We’ve listed some of the most common repair methods that you are likely to come across:

  • Drop leaking tubes from the circuit: Keep in mind however that every dropped tube reduces the coil’s performance by triple the surface area of the tube that is dropped. Again, while ok in the short-term, this is simply another “Band-Aid” fix. Over time, your energy costs will rise exponentially, and you will probably end up buying a new coil anyway.
  • Braze over the existing braze: As mentioned above, erosion has caused the original braze to fail, so all that you are really doing is pushing the pressure to another braze, which will then begin to fail as well.
  • High Pressure Cleaning: This method bends the fins, further restricts the airflow, and pushes dirt more to the center of the coil, which can never be adequately cleaned.

The real reason why coils need to be replaced rather than repaired is due to energy costs. If your coil is not operating near desired levels, you’ll need to increase the energy to make it work at its peak performance. Energy increases might be slight at first, but they are guaranteed to continue to rise over time. For example:

  • Somebody adjusts the fan drive for higher speeds, higher CFM’s and higher BTU’s.
  • Someone adjusts the boiler; the water and steam temperatures are higher.
  • Someone adjusts the chiller (1) degree higher for colder water to the chilled water coil.

Whichever method is used, performance begins to suffer and adjustments to the system occur. These adjustments cost energy efficiency and ultimately, money!

If you have ever experienced repairing a coil, then you know it is labor intensive and typically will not work as a permanent solution. With very few exceptions, repairs should be seen as nothing more than temporary until you’re able to replace that coil!

Capital Coil & Air has seen every “repair” method used, as well its inevitable outcome, so instead of putting yourself through that, call Capital Coil and allow us to be your coil replacement experts.

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Case Study: We Need These Coils on a (5) Day Quick-ship

In late June, Capital Coil received a call from a Trane office in Ohio regarding quick-ship availability. One of Trane’s top customers had an urgent need for (12) large chilled water coils with stainless steel casing. The problem/hurdle that they were encountering was that they needed all (12) coils to be built and ship out of the factory in (5) business days. Completion of the whole project was 100% contingent on them receiving the coils in their specified time-frame. An additional complication was the fact that July 4th was the following week, and they needed to have the coils ship prior to the holiday.

Trane shopped the project around to different manufacturers, but not one could guarantee to ship in (5) days. Some manufacturers waffled and claimed that they could have them built in (6) or (7) days, but not one could guarantee to ship in (5) days. A sales rep in that same Trane office, who had worked with Capital Coil previously, suggested that his co-worker reach out to us to see what we could do. After speaking with Trane’s project manager, we immediately contacted our head of production to make sure that we had the capacity to complete all (12) coils in the required (5) days. She assured us that we had the materials and manpower on-hand to get them all built and ship on time. We agreed to accept the project and began work on the coils immediately.

Due to the size of the project, as well as it’s time-sensitivity, we had multiple calls daily with our factory to ensure that everything was proceeding on schedule. We then gave Trane daily status updates, so they were constantly informed of everything from the brazing of the coils to entering the final testing phase. Chilled Water Coil

As promised, all (12) coils were built correctly and shipped out in the required (5) days. Our logistics team was then in constant communication with the freight company to make sure that the delivery was on schedule. And just like during the production phase, we passed daily tracking updates along to Trane, so they knew where their coils were at all times and when they could expect delivery. All (12) chilled water coils arrived on July 3rd with zero freight damage, and the project was completed on time!

A company as large and influential as Trane can have their coils built by anyone, but Capital Coil was the only manufacturer that could guarantee to have their coils built and shipped by a required date. Additionally, in working so closely with Trane throughout the whole process, they were kept up-to-date in real time from the start of production to final delivery.

Capital Coil offers a level of service that you won’t get with other manufacturers. When we guarantee to ship by a certain date, we stand by that guarantee, or you do not pay!

 

Trane’s project manager’s comments to Capital Coil upon completion/delivery:

“This will help us get a jump on this project prior to the big event taking place next week! 

I will make sure to share your information with others across our great lakes region about our experience with your company, so that they know we have THIS option to go to for our coil needs. THANK YOU ALL!!”

Steam Distributing Coils (Non-Freeze)

Steam Distributing CoilsWere you aware that Steam Distributing coils or “Non-Freeze” steam coils were essentially discovered by accident? First, it must be mentioned that there is no such thing as a 100% “Non-Freeze” steam coil because under the right conditions, any coil can freeze. As such, Capital Coil tries to steer clear of the term “Non-Freeze” because it is a mischaracterization. Steam Distributing Coils is the correct terminology that Capital Coil uses when speaking about steam coils that see entering air temperatures under 32* F. Trapped condensate in the tubes and/or headers, coupled with entering air temperatures below 32*F over the face of the coil, creates a situation with a near-100% certainty that your steam coil will freeze. Because of this, there is no magical solution to fully eliminate freezing your coil, which again is why Capital Coil does not use the term “Non-Freeze”.

Steam turns to condensate little by little as it travels through the coil. Lower pressure steam turns to condensate faster than higher pressure steam!! The longer the tube length in the coil, the earlier the condensate is formed, and the longer it has to travel through the tubes. One very important fact to always remember is that too much condensate in a steam coil IS NEVER A GOOD THING…under any circumstances! Because of this requirement, everything is designed to ensure the removal of all condensate from the coil. Systems are heavily designed with float & thermostatic traps, vacuum breakers, and placement of piping to help get rid of any remaining condensate.

Another headache that occurs when condensate freezes is that it creates a “water-hammer”. A “water-hammer” can best be described as a loud banging noise as the steam is coming into contact with the condensate in the coil. It does not allow the steam to be evenly distributed across the face of the coil…again not a good thing!

At the inception of the HVAC industry, steam coils were originally designed to be shorter in length because there was not a good way to evacuate condensate. In an effort to make steam coils longer in length, the concept of a steam coil containing a tube within a tube was invented. The steam feeds only the inner tubes, which travels the entire of the length of the outer-tube. Holes are placed every 12” with the inner tube releasing condensate to the outer-tube. The idea is that the condensate is slowly and evenly “distributed” across the entire length of the coil. Heating is also evenly applied across the coil’s face, and if the casing is pitched at a downward angle, condensate cannot remain trapped. It was later discovered as an added bonus that under most circumstances these coils will not freeze. So while the concept was never designed or intended to become known as “Non-Freeze”, they are now used in almost all projects dealing with air temperatures below 32*F. Please keep in mind that you will still need all of the other steam protective devices in the system, including the freeze-stat, but all in all, it is much more difficult to freeze coils today than it was 30-40 years ago. Necessity may be “the mother of invention” but this great concept was discovered accidently.

Capital Coil is available for all of your coil-related trivia needs, so please don’t hesitate to reach out whenever we can be of assistance.

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Steam Distributing Coils

Guidelines For Air Velocities

The height, length and resulting air velocities greatly figure in everything in determining the size and performance of a coil. Step # 1 in determining the size and performance of a coil is dependent upon understanding face & air velocities of air across the coil. Whether you use CCA’s coil selection program to help size the coil, or you are replacing an existing coil; the height, length and resulting velocity determine everything.

Hot Water Booster Coils

air velocities

Every coil has a specific, optimum velocity, so you want to make sure you are within 30% (+ or -) of that number. For example, booster coils have an optimum velocity of 800 ft/minute. That means that you can drop your velocity to 600 ft/minute, or conversely, increase the velocity to 1,000 ft/minute. The duct velocities are almost always higher, which means that you will need to transition to a larger coil. Try to get to as close to 800 ft/minute as possible, while sizing your coil to make the transition as easy as possible. Everything with coils is a balancing act.

Hot Water & Steam Coils

Like booster coils, hot water and steam coils should also have face velocities at approximately 800 ft./minute. Both steam & hot water coils have only sensible heating, which is why their face velocities can be the same. Face velocities ultimately control the coil’s cost, so 800 ft./minute really is a heating coil’s “sweet spot”.

If you are purchasing an air handler unit, oftentimes the heating coil is smaller than the cooling coil because the face velocities on heating coils can exceed those of cooling coils. Due to water carry-over, cooling coils cannot exceed 550 ft/minute, while heating coils only deal with sensible heat.

Chilled Water & DX Coils

Due to the limited face velocities of cooling coils, your choices are more limited. With cooling coils, your face velocity must be somewhere between 500 ft./minute-550 ft./minute. Remember that when dealing with cooling coils, you are dealing with both sensible and latent cooling, so the coil is wet. When you exceed 550 ft./minute, water carry-over occurs past the drain pans.

If you are purchasing an air handler unit, you probably will not have worry about the coil’s face velocity as most coils come pre-sized at the acceptable face velocities. Fan coils also come pre-sized with the correct CFM’s. However, if you are replacing an existing cooling coil, the face velocity must remain at or below 550 ft/minute!!

 Air Stratification Across The Coil

Air does not travel equally across the face of a coil. If you were to divide a coil into (9) equal sections, like a tic-tac-toe board, you would see a high percentage of air travelling through the center square, rather than the corner squares. In a perfect air flow scheme, 11% of the air would travel through each of the 9 squares, but that is not what happens. Because more air travels through the center of the coil, you want to avoid putting a fan too near the coil. Due to central air flows, most systems are draw-thru, rather than blow-thru. This is also why you want to avoid installing your coil near any 90 degree angles/turns in the ductwork. Avoid any situations that contribute more than the “natural” air stratification to help ensure your coil is at maximum efficiency.

In some situations involving cooling coils, you will have water carry-over even when the coil is sized correctly. How can this happen? Think about the tic-tac-toe board again. Air velocities are exceeding 700 ft./minute in the coil’s center, while the corners are around 300 ft./minute. This cannot and will not work.

Coils do not have any moving parts. They simply react to the air across the outside of the coil and whatever is running through the inside of the coil. Coils are 100% a function of your entire system, as well as the installation in general.

Capital Coil & Air is here to help with any coil selections that will help avoid costly missteps that lead to wasted time and money. Call us on your next project, we greatly look forward to working with you!

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